A group of enzymes that oxidize primary monoamines but have little or no activity towards diamines, such as histamine, or towards secondary and tertiary amines. They are copper quinoproteins (2,4,5-trihydroxyphenylalanine quinone) and, unlike EC 1.4.3.4, monoamine oxidase, are sensitive to inhibition by carbonyl-group reagents, such as semicarbazide. In some mammalian tissues the enzyme also functions as a vascular-adhesion protein (VAP-1).